You should tell her you are an atheist and that she should respect your decision to not believe in an imaginary sky cretin, if she gets 'aggressive' then just ignore her words. If she tells your family then she has done something you should've done a long time ago. If your family do not respect your atheism straight away I'm sure they will eventually...if they don't after a very long time perhaps they don't deserve to be your family.
"God is dead" - Friedrich Nietzsche
"Faith is what you have in things that DON'T exist. - Homer J. Simpson
"Faith is what you have in things that DON'T exist. - Homer J. Simpson